As of March 2PM Eastern time, the official Mozilla Firefox ESR site
is still offering ESR 45.8.0 and NOT esr 52.0.0
Please notice that TFA links to their own download site and NOT Mozilla's
As per the NPAPI support:
NPAPI support is there in the code, since the NPAPI Flash plug-in still works. Is only that Mozilla's developers decided to disable it for all other plugins.
Plugins that do not use NPAPI are failing because Firefox is slowly rolling out multiplrocess (project electrolysis) and this interferes with Plugins.
As for Ad-ons (which are different beasts than plugins), the problem is both project Electrolysis AND the fact that Mozilla is migrating from their plug-in APIs of yore, to an API similar (but no completely equal to) chrome's, for security and performance reasons.
The path of least resistance, at least for now, is to install ESR 52, disable multiprocess and hang on to it until around june next year. Also, bear in mind that, on older hardware, Multiprocess (think core processos before the i series, specialy 2 cores non multithreaded machines) is actually SLOWER than the singlethreaded firefox way. This is specially important for Plugins used to handle pro grade equipment (servers, networking gear, etc)
The harder, but more efective long term path is to upgrade/substitute problematic Plug-Ins and AdOns and embrace the more secure multiprocessing future head on. Or change browser...
Me, I have been on the ESR channel since it was enabled, so you now my answer... ;-) :-P